Last month a Chinese official in charge of internet surveillance gave notice that mobs of web users who turn on individuals and make their lives a misery will not be tolerated. In China it happens often and on a massive scale, earning the phenomenon the title of the "human flesh search engine".

On Thursday 21 March 2013, the world changed for Yin Feng, a self-described "average guy" who worked as a part-time taxi driver in the western Chinese city of Urumqi.

Just after 14:00 his mobile phone began ringing off the hook. The callers all berated the bewildered Yin, screaming obscenities and accusing him of acting like an animal.

It took a while for Yin to uncover why the strangers phoning him were so upset.

"Finally, some sensible citizens told me a story about me they heard on the radio, or on the internet," he recalls.

Earlier that day, they told him, a driver in Urumqi had rolled down his window to spit on an elderly homeless person lying on the street. Witnesses recorded the first few digits of the spitter's number plate. The information was quickly broadcast by a local radio station.

Thousands then banded together online to track down the perpetrator.

Yin Feng holds a piece of paper protesting his innocence

"Driver with the licence plate A36D62, you really humiliate all men," wrote one angry internet user. "Please forward this post and let's see what kind of ugly face he has. Let's extinguish him. Die! Such a disgrace. We don't even know where he's from. Get out of Urumqi."

Hours later, they zeroed in on Yin, whose number plate was a partial match, and posted his mobile number online.

The internet vigilantes were wrong, Yin insists. He tried to defend himself to anyone who would listen, explaining he wasn't guilty of spitting on anyone.

But as soon as Yin hung up with one angry caller, his phone would ring again. And again. And thousands of times again.

The case of Wang Fei

In 2008, a Beijing widower named Wang Fei found himself under attack from the human flesh search engine. His wife had killed herself after finding out he had been having an affair. A friend then hacked into her email and posted her diaries online.

Within days, Wang had lost his job and had moved in with his parents to escape angry internet users who had read the diaries. But he was followed - mobs painted death threats on his parents' door and handed fliers with his photo, calling him a murderer.

"I told him we could only sue websites and people with identities. Those internet users, tens of thousands of them, we don't know who they are," says his lawyer, Zhang Yanfeng.

In the end, Wang successfully sued the operators of three websites for violating his privacy and unfairly smearing his reputation.

"All of my private information was made public. My ID card number, name, phone number, address, even my mother-in-law's phone number was dug out and posted online," Yin remembers. "I even received phone calls blackmailing me, threatening to burn my house down if I didn't pay them 200,000 RMB [$33,000; £20,000]."

Mr Yin was an unwitting target of what has been named China's human flesh search engine.

At its worst, the ghoulishly named "flesh-searching" phenomenon is cyberbullying on an epic scale, sometimes involving hundreds of thousands of anonymous Chinese internet surfers ganging up to uncover the identity of an unsuspecting target. Users band together to uncover a person's identity - sometimes a suspected adulterer, sometimes an animal abuser.

The phenomenon first scored attention in 2006, when many in China began to turn to the internet for the majority of their entertainment. The use of internet forums exploded during that period.

One of the first notorious cases involved the search for a woman who starred in an anonymous video using the high heel of her stiletto to crush a kitten's skull. The woman, who turned out to be a nurse, was suspended from her job when her identity was revealed. She received numerous death threats and considered suicide, Chinese state television reported.

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Numerous government officials have been forced to resign after their luxurious spending habits were discovered by curious internet users”

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Hundreds of similar cases followed. With 591 million users, China has the world's largest internet population. They're also arguably the most obsessive, picking over the smallest details in photos that capture the public's imagination.

But at the highest levels, it seems the government is taking notice. Last month, Liu Zhengrong, a top Communist official in charge of China's internet surveillance, said the government believed the human flesh search engine was "illegal and immoral". His caution was soon echoed in China's major state media outlets - a signal, Chinese lawyers say, that flesh-searching tactics won't be tolerated in the courts. Legislation might soon follow.

But up until now it has been possible for anyone to find themselves in the crosshairs of China's internet forums.

In 2009, Zhang Zetian was an ordinary high school student. One day, as she was leaving class, a friend snapped a photo of her with a Chinese milk tea drink in her hand, backpack slung over one shoulder. Zhang's photo was then posted on Renren.com, a popular social networking site. Complete strangers then forwarded the photo hundreds of thousands of times, proclaiming the "Milk Tea Girl" to be "adorable!" and "fresh faced!".

Zhang Zetian, the "Milk Tea Girl"

"A newspaper reporter called me one day and suddenly I realised that people had noticed me on the internet," Zhang explains. All her personal details were posted online.

Years later, Zhang remains an internet celebrity. Photos of her doe-eyed face are in regular circulation. When she was admitted into Tsinghua, one of China's top universities, her profile rose again.

Sitting in a cafe near her campus, Zhang seems embarrassed by her unlikely rise to celebrity status.

"No matter where I go, people attempt to take secret photos of me," she says. People follow her with cameraphones on campus and sometimes in class. Admirers have even tried to break into her university dormitory.

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Even if laws appear on the books, Beijing might find it difficult to reign in the public's appetite for scrutinising others”

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Those who favour increased policing of the internet to stop flesh-searching cite cases like the Milk Tea Girl as classic examples. When so many strangers focus their attention on a single person, some inevitably go too far.

Sceptics might point to the fact that it's in the government's interest to rein in the practice, since the flesh-searching phenomenon has also targeted members of the Communist Party.

The most famous case of political flesh-searching involves the "watch uncle", a Communist official in China's central Shaanxi province who was spotted smiling at the scene of a deadly traffic accident. Who was this man, many wondered.

They soon discovered he was the province's health and safety chief, a man named Yang Dacai. Some also realised that in every official photograph, Yang was wearing a different designer watch, worth far more than he could afford on his official salary. Days after his smiling face first snagged attention, he found himself without a job.

Since then, watch spotting has become an internet sport and "watch uncles" have been outed all over China. Some officials even attracted attention when their obvious tan lines indicated they had just removed their watches before allowing a photo to be taken.

A "watchless" official

In small ways, the human flesh search engine is forcing Communist officials to change their behaviour.

By using the internet to police the party, citizens can train their own government to obey the constitution, argues Wu Zuolai, a scholar with the Chinese Academy of Arts in Beijing.

"They get criticised every day, and it will become a regular routine," he says. "Before, leaders locked themselves up in Zhongnanhai [the government leaders' compound], focusing their minds on how to keep the people in sealed boxes without speaking or moving freely. I believe Xi Jinping's era will be more open."

Even if laws appear on the books, Beijing might find it difficult to reign in the public's appetite for scrutinising others.

The internet is notoriously hard to control, even in China, where censors regularly delete blog posts and comments the government deems unacceptable. Some targets of flesh-searching have already taken their cases to court, but it's hard to blame a single perpetrator.

Months later, Yin Feng, the taxi driver, is still shaken by his ordeal. He scans the internet regularly for mentions of his name and he watches other flesh-searching incidents carefully. Attempts to report his experience to the authorities have not had much effect.

Yin hopes the government will enact new laws to give ordinary folk like him power against internet vigilantes.

"Many years have passed since the internet became so powerful," he explains. "If other victims' personal lives are affected like mine was, at least they'll have the law to turn to. If nothing is done, frightening things will happen."

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